Bump manufacturing method

ABSTRACT

A method of forming bumps on the active surface of a silicon wafer. An under-ball metallic layer is formed over the active surface of the wafer. A plurality of first solder blocks is attached to the upper surface of the under-ball metallic layer. Each first solder block has an upper surface and a lower surface. The lower surface of each first solder block bonds with the under-ball metallic layer. The upper surfaces of the first solder blocks are planarized. A second solder block is attached to the upper surface of each first solder block and then a reflow operation is carried out.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of Taiwan applicationserial no. 91102992, filed Feb. 21, 2002.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to a method of manufacturing bumps. Moreparticularly, the present invention relates to a bump manufacturingmethod capable of strengthening the bond between a solder block and asilicon wafer.

2. Description of Related Art

In this information explosion age, electronic products are used almosteverywhere. Computer and processing stations driven by powerfulintegrated circuits are employed in offices, educational institutions,recreational industries, business and commercial companies. Aselectronic technology continues to progress, products having morepowerful functions and more attuned to personal needs are developed.Furthermore, most electronic products are increasingly light and compactthanks to the efficient fabrication of many types of high-densitysemiconductor packages. A major innovation is the flip chip designcapable of cramming a considerable number of integrated circuitstogether. In a flip-chip design, a plurality of bumps is formed on thebonding pads of a silicon chip. Each bump directly contacts acorresponding contact point on a substrate so that the chip and thesubstrate are electrically connected. Compared with conventional wirebonding and tape automated bonding (TAB) methods of joining a chip witha substrate, the flip-chip design has a shorter overall conductive pathand hence a better electrical connectivity. In addition, the backside ofthe chip may be exposed to facilitate heat dissipation during operation.Due to the distinguishing advantages of flip-chip packages,semiconductor manufacturing favors its production.

FIGS. 1 to 4 are partially magnified cross-sectional views showing thesteps for forming bumps on a silicon wafer according to a conventionmethod. As shown in FIG. 1, a silicon wafer 110 having an active surface112 is provided. The wafer 110 also has a passivation layer 114 and aplurality of bonding pads 116 (only one is shown) over the activesurface 112 of the wafer 110. The passivation layer 114 exposes thebonding pads 116. A conventional stud-forming machine is used to form afirst solder block 120 over each solder pad 116. The first solder blocks120 are made from a material such as copper or gold. The upper surfaces122 of the first solder blocks 120 are planarized to form a structure asshown in FIG. 2.

As shown in FIG. 3, a conventional wire-bonding machine is used toattach a second solder block 130 to the upper surface of the firstsolder block 120. The second solder blocks 130 are made from a materialsuch as lead-tin alloy. A reflow operation is carried out sprinkling aflux material over the wafer and heating the wafer. The heat softens thesecond solder blocks 130 and transforms the second solder blocks 130into blobs of material having a hemispherical profile as shown in FIG.4. This completes the fabrication of a bump 140 (only one is shown)comprising one first solder block 120 and one second solder block 130.

In the aforementioned fabrication process, the first solder block 120directly bonds with the bonding pad 116. Hence, the first solder block120 and the bonding pad 116 must have good bondability. However, notevery type of material constituting the first solder block 120 has thecapacity to wet the bonding pad 116 material. Thus, there is limitationin the selection of material forming the first solder blocks 120.Improper selection of first solder block material may result in theformation of a weak bond with the bonding pads 116. Furthermore, some ofthe first solder block material such as copper has great diffusioncapacity. Such metallic particles may diffuse into the wafer formingunwanted conductive circuits between metallic interconnects inside thewafer. In some cases, the chip may fail because of this, leading to alower production yield.

SUMMARY OF INVENTION

Accordingly, one object of the present invention is to provide a methodof forming bumps over a silicon wafer such that an additional under-ballmetallic layer is formed between a bonding pad on the wafer and a firstsolder block. Hence, ultimate adhesive strength of the bump withcorresponding bonding pad is increased.

A second object of this invention is to provide a method of formingbumps over a silicon wafer such that an additional under-ball metalliclayer is formed between a bonding pad on the wafer and a first solderblock. Hence, the diffusion of metallic particles inside the bump intothe wafer is prevented and the probability of chip failure due to shortcircuit is reduced.

Note in the following description that the use of the preposition “over”as in “a second layer is formed over a first layer” means that thesecond layer is either in contact with the first layer or simply abovethe first layer.

To achieve these and other advantages and in accordance with the purposeof the invention, as embodied and broadly described herein, theinvention provides a method of forming a plurality of bumps over asilicon wafer. The wafer has an active surface with a passivation layerand a plurality of bonding pads thereon. The passivation layer exposesthe bonding pads. First, an adhesion layer is formed over the activesurface of the wafer covering the bonding pads and the passivationlayer. A barrier layer is formed over the adhesion layer. A wettablelayer is formed over the barrier layer.

A photolithographic process is conducted to from a plurality ofphotoresist blocks over the wettable layer. A first etching operation iscarried out to remove the wettable layer, the barrier layer and theadhesion layer outside the photoresist blocks so that only the wettablelayer, the barrier layer and the adhesion layer underneath thephotoresist blocks remain. The photoresist blocks are removed.

A first solder block bonds onto the wettable layer through a bondingoperation. Each first solder block has an upper surface and a lowersurface. The lower surface of the first solder block bonds with thewettable layer. The upper surface of the first solder block isplanarized through polishing. Thereafter, a second solder block bondsonto the upper surface of the first solder block through another bondingoperation. A reflow operation is next carried out.

According to the embodiment of this invention, the adhesion layer can bemade from a material including titanium, titanium-tungsten alloy,aluminum or chromium. The barrier layer can be made from a materialincluding nickel-vanadium alloy, chromium-copper alloy or nickel. Thefirst solder block can be made from a material including lead-tin alloy,tin-silver alloy, tin-silver-copper alloy, silver or gold. The secondsolder block can be made from a material including lead-tin alloy,tin-silver alloy, tin-silver-copper alloy or tin.

In addition, after bonding a second solder block onto a first solderblock, the upper surface of the second solder block may be planarizedthrough polishing. The reflow operation is carried out after theplanarization. However, the process of planarizing the upper surface ofthe second solder block can also be omitted entirely.

In brief, because the first solder block is bonded onto the wettablelayer, a material capable of wetting the first solder block may bechosen as the material constituting the wettable layer. Hence, the firstsolder blocks are tightly coupled to the wafer. The under-ball metalliclayer may be designed according to the material constituting the firstsolder blocks so that solder blocks of whatever material can attachfirmly to the active surface of the wafer. Furthermore, through theplacement of an under-ball metallic layer, diffusion of metallicparticles from the solder block to the wafer is blocked. Hence, thediffusion of metallic particles into the insulation layer of the waferleading to chip failure is greatly minimized. Moreover, different typesof materials may be used to form the first solder blocks and the secondsolder blocks and the volume of material forming the first solder blocksand the second solder blocks may be tailor-made by controlling thesettings of a wire-bonding machine. The bulk of the first solder blocksand the second solder blocks can further be controlled through polishingso that solder blocks having a variety of ratios are possible.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings,

FIGS. 1 to 4 are partially magnified cross-sectional views showing thesteps for forming bumps on a silicon wafer according to a conventionmethod; and

FIGS. 5 to 16 are partially magnified cross-sectional views showing thesteps for forming bumps on a silicon wafer according to one preferredembodiment of this invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

FIGS. 5 to 16 are partially magnified cross-sectional views showing thesteps for forming bumps on a silicon wafer according to one preferredembodiment of this invention. As shown in FIG. 5, a silicon wafer 310having an active surface 312 with a passivation layer 314 and aplurality of bonding pads 316 (only one is shown) thereon is provided.The passivation layer 314 exposes the bonding pads 316.

As shown in FIG. 6, an adhesion layer 320 is formed over the activesurface 312 of the wafer 310 by sputtering. The adhesion layer 320covers both the bonding pads 316 and the passivation layer 314. Theadhesion layer is made from a material such as titanium,titanium-tungsten alloy, aluminum and chromium. A barrier layer 330 isformed over the adhesion layer 320 by sputtering or electroplating. Thebarrier layer 330 is made from a material such as nickel-vanadium alloy,chromium-copper alloy or nickel. A wettable layer 340 is formed over thebarrier layer 330 by sputtering or electroplating. The wettable layer340 is made from a material such as copper, palladium or gold. Thiscompletes the fabrication of an under-ball metallic layer 342. Theunder-ball metallic layer 342 actually comprises the one adhesion layer320, one barrier layer 330 and one wettable layer 340.

As shown in FIG. 7, a photolithographic process is carried out to form aphotoresist layer over the wettable layer 340. The photoresist layer isexposed to light and chemically developed to transfer a pattern (notshown) to the photoresist layer. Hence, a plurality of photoresistblocks 350 (only one is shown) is formed in locations where bumps needto be formed. In fact, the photoresist blocks 350 are formed directlyover the bonding pads 316. An etching operation is conducted to removedthe exposed wettable layer 340, the barrier layer 330 and the adhesionlayer 320 so that only the wettable layer 340, the barrier layer 330 andthe adhesion layer 320 underneath the photoresist blocks 350 remain asshown in FIG. 8. The photoresist blocks 350 are removed to form astructure as shown in FIG. 9.

As shown in FIG. 10, a first solder block is formed on each bonding pad316 of the wafer 310. A conventional stud-forming machine is used toconduct the bonding operation. The stud-forming machine has awire-bonding head 360 with a capillary channel 362 for accommodating aconductive wire 364. The conductive wire 364 is free to slide inside thecapillary channel 362. A spike discharge method is used to generate heatat one end 366 of the conductive wire 364 so that the wire 364 partiallymelts. Through the inward pull of metallic ions, the heated end of thewire forms a spherical blob 368. While the spike discharge operation iscarried out, the end of the conductive wire 364 is immersed in an inertatmosphere so that the heated blob 368 is prevented from oxidation.

As shown in FIG. 11, the material blob 368 at the end of the conductivewire 364 moves down and contacts the wettable layer 340 before the blob368 solidifies. Ultrasound is also applied concurrently to assist thejoining of the spherical block 368 with the wettable layer 340. Thespherical blob 368 and the wettable layer 340 wet each other so that thespherical blob 368 is firmly attached to the under-ball metallic layer342. Thereafter, the wire-bonding head 360 is raised so that theconductive wire 364 detaches from the spherical blob 368 to form astructure as shown in FIG. 12. This completes the attachment of thefirst solder blocks 370 (only one is shown). The first solder block 370has an upper surface 372 and a lower surface 374. The lower surface 374of the first solder block 370 bonds with the wettable layer 340. Thefirst solder blocks 370 are made from a material such as 63Sn/37Pblead-tin alloy, 90Pb/10Sn lead-tin alloy, 95Pb/5Sn lead-tine alloy,97Pb/3Sn lead-tin alloy, 95Sn/5Ag tin-silver alloy, 97.5Sn/2Ag/0.5Cutin-silver-copper alloy, 96.5Sn/3.5Ag tin-silver alloy, silver or gold.A polishing operation or a pressurizing operation is conducted toplanarize the upper surface 372 of the first solder block 370 so thatthe upper surface 372 of the first solder block 370 becomes planar asshown in FIG. 13. Through the polishing operation, volume of firstsolder block material on the under-ball metallic layer can becontrolled.

As shown in FIG. 14, a second wire-bonding operation is conducted toform a plurality of second solder blocks 380 (only one is shown) on theupper surface 372 of various first solder blocks 370. A method similarto the one forming the first solder blocks 370 is used and hencedetailed description is omitted. The second solder blocks 380 are madefrom a material such as 63Sn/37Pb lead-tin alloy, 90Pb/10Sn lead-tinalloy, 95Pb/5Sn lead-tine alloy, 97Pb/3Sn lead-tin alloy, 95Sn/5Agtin-silver alloy, 97.5Sn/2Ag/0.5Cu tin-silver-copper alloy, 96.5Sn/3.5Agtin-silver alloy or tin. A reflow operation is carried out sprinklingflux material over the wafer 310 and heating the wafer so that the firstsolder blocks 370 and the second solder blocks 380 melt together to formsolder blocks 390 (only one is shown). Therefore, a set of bumps 392(only one is shown) each comprising one solder block 390 and oneunder-ball metallic layer 342 having a structure shown in FIG. 15 isformed. Note that the reflow temperature must be higher than thealloying temperature between the first solder block 370 and the secondsolder block 380. Finally, the wafer 310 is sliced up into a pluralityof chips 318 as show in FIG. 16.

In the aforementioned processes, the first solder block 370 is attachedto the wettable layer 340. Hence, as long as the wettable layer 340 ismade from a material capable of wetting the first solder blocks 370, thefirst solder blocks 370 are firmly attached to the wafer 310.Furthermore, the under-ball metallic layer 342 may be designed accordingto the material selected to form the first solder blocks 370. In thisway, the solder blocks 370 are able to hold firmly to the active surface312 of the wafer 310 whatever the type of material constituting thesolder blocks 370. In addition, the under-ball metallic layer 342 servesalso as a barrier blocking the diffusion of metallic particles into theinsulation layer inside the wafer 310 that may lead to chip failure.

The first solder blocks 370 and the second solder blocks 380 can befabricated using a different material. For example, the first solderblocks 370 are all made from 63Sn/37Pb lead-tin alloy while the secondsolder blocks 380 are all made from 97Pb/3Sn lead-tin alloy.Consequently, a lead-tin alloy having a specified lead/tin ratio such as70Pb/20Sn may be produced after a reflow operation. Therefore, lead-tinalloy of various ratios may be produced in this way. Moreover, theaforementioned processes can also be applied to the fabrication of leadfree bumps. For example, silver is used to form the first solder blocks370 while tin is used to form the second solder blocks 380. Thus, atin-silver alloy having a specified tin/silver ratio such as 95Sn/5Agmay be produced after a reflux operation. Hence, tin-silver alloy ofvarious ratios may be produced as well.

Furthermore, the upper surface 382 of the second solder blocks 380 maybe polished to obtain a planar top. Through the polishing operation,volume of second solder block material on top of the first solder block370 may also be modified. in fact, volume of material constituting thefirst solder blocks 370 and the second solder blocks 380 can beprecisely controlled through separate polishing of the upper surface 372of the first solder blocks 370 and the upper surface 382 of the secondsolder blocks 380 respectively. Thus, the metallic composition of thebumps 390 after the reflow operation can be precisely controlled. Ingeneral, aside from the wire-bonding operations, volume of material ineach first solder block 370 and second solder block 380 can be preciselycontrolled through polishing to produce bumps having a differentcompositional ratio.

The smallest distance of separation capable of being produced by awire-bonding machine is currently down to 40 μm. Accordingly, minimumdistance of separation between neighboring bumps 392 permissible is alsoabout the same magnitude.

In general, other types of under-ball metallic material may be used inthe fabrication of the bumps aside from the aforementioned materials.Moreover, the bonding pads may be made from a material such as aluminumor copper.

The under-ball metallic layer according to this invention need not belimited to just three layers (the adhesion layer, the barrier layer andthe wettable layer). Other number of conductive layers is possible. Forexample, the under-ball metallic layer can be a structure with fourlayers, including a chromium layer, a chromium-copper alloy layer, acopper layer and a silver layer. Alternatively, the under-ball metalliclayer can be a structure with two layers, including a lower layer suchas a titanium-tungsten alloy layer or a titanium layer and an upperlayer such as a copper layer, a nickel layer or a gold layer.

In addition, the wafer may be sliced up before attaching the firstsolder blocks and the second solder blocks onto the active surface in awire-bonding operation and performing a reflow operation. Furthermore,the first solder blocks may bond with the under-ball metallic layerimmediately after forming the under-ball metallic layer. Thereafter, theunder-ball metallic layer is etched using the first solder blocks as anetching mask. With this arrangement, one photolithographic step issaved.

Although the bumps are directly formed on the active surface of asilicon wafer in the aforementioned embodiments, the bumps may also formelsewhere. For example, the bumps may form over a redistribution layerafter the redistribution layer is formed on a silicon wafer.

In conclusion, the method of forming bumps on a wafer according to thisinvention has at least the following advantages:

1. The first solder block is attached to the wettable layer. Hence, aslong as the wettable layer is made from a material capable of wettingthe first solder blocks, the first solder blocks are firmly attached tothe wafer. Furthermore, the under-ball metallic layer may be designedaccording to the material selected to form the first solder blocks. Inthis way, the solder blocks are able to hold firmly to the activesurface of the wafer whatever the type of material constituting thesolder blocks.

2. The under-ball metallic layer serves also as a barrier blocking thediffusion of metallic particles into the insulation layer inside thewafer. Hence, chip failure due to short circuit is prevented.

3. Different types of material may be employed to form the first solderblocks and the second solder blocks and volume of material used by eachtype of solder blocks may be precisely controlled. Hence, bumps havingany type of compositional ratio may be produced.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

What is claimed is:
 1. A method of forming bumps on a silicon waferhaving an active surface with a passivation layer and a plurality ofbonding pads thereon, wherein the passivation layer exposes the bondingpads, the method comprising the steps of: forming an adhesion layer overthe active surface of the wafer, wherein the adhesion layer covers boththe bonding pads and the passivation layer; forming a barrier layer overthe adhesion layer; forming a wettable layer over the barrier layer;conducting a photolithographic process to form a plurality ofphotoresist blocks over the wettable layer; conducting an etchingoperation to remove the wettable layer, the barrier layer and theadhesion layer outside the photoresist blocks so that only the wettablelayer, the barrier layer and the adhesion layer underneath thephotoresist blocks remain; removing the photoresist blocks; attaching aplurality of first solder blocks to the upper surface of the wettablelayer through a bonding operation, wherein each first solder block hasan upper surface and a lower surface such that the lower surface of thefirst solder block bonds with the wettable layer; planarizing the uppersurface of the first solder blocks; attaching a second solder block tothe upper surface of each first solder block through a bondingoperation; and alloying the first solder block and the second solderblock to attain a single solder bump.
 2. The method of claim 1, whereinmaterial constituting the adhesion layer is selected from a groupconsisting of titanium, titanium-tungsten alloy, aluminum and chromium.3. The method of claim 1, wherein material constituting the barrierlayer is selected from a group consisting of nickel-vanadium alloy,chromium-copper alloy and nickel.
 4. The method of claim 1, whereinmaterial constituting the wettable layer is selected from a groupconsisting of copper, palladium and gold.
 5. The method of claim 1,wherein each second solder block includes an upper surface and a lowersurface, the lower surface is in contact with the upper surface of thefirst solder block, and the upper surface of the second solder block isplanarized by polishing.
 6. The method of claim 1, wherein materialconstituting the first solder blocks is selected from the groupconsisting of lead-tin alloy, lead-silver alloy, tin-silver alloy,silver and gold.
 7. The method of claim 1, wherein material constitutingthe second solder blocks is selected from the group consisting oflead-tin alloy, tin-silver alloy, tin-silver-copper alloy and tin. 8.The method of claim 1, wherein the step of planarizing the upper surfaceof the first solder blocks includes polishing.
 9. The method of claim 1,wherein the step of planarizing the upper surface of the first solderblocks includes applying a pressure.
 10. The method of claim 1, whereinthe step of attaching a first solder block to the wettable layerincludes the sub-steps of: providing a conductive wire; heating one endof the conductive wire so that the heated end of the conductive wiretransforms into a spherical blob; pulling the spherical blob towards thewettable layer and pressing the spherical blob against the surface ofthe wettable layer; and detaching the remaining portion of theconductive wire from the spherical blob to form the first solder block.11. The method of claim 10, wherein the step of pressing the sphericalblob against the wettable layer is further assisted by application ofultrasound.
 12. The method of claim 1, wherein the step of attaching asecond solder block to a first solder block further includes thesub-steps of: providing a conductive wire; heating one end of theconductive wire so that the heated end of the conductive wire transformsinto a spherical blob; pulling the spherical blob towards the firstsolder block and pressing the spherical blob against the upper surfaceof the first solder block; and detaching the remaining portion of theconductive wire from the spherical blob to form the second solder block.13. The method of claim 12, wherein the step of pressing the sphericalblob against the first solder block is further assisted by anapplication of ultrasound.
 14. A method of forming bumps on an activesurface of a silicon wafer, the method comprising the steps of: formingan under-ball metallic layer over the active surface of the wafer,wherein the under-ball metallic layer is a composite layer; removing aportion of the under-ball metallic layer to expose the active surface ofthe wafer; attaching a plurality of first solder blocks to an uppersurface of the under-ball metallic layer, wherein each first solderblock has an upper surface and a lower surface such that the lowersurface of each first solder block bonds with the under-ball metalliclayer; planarizing the upper surface of the first solder blocks;attaching a second solder block to the upper surface of each firstsolder block; and transforming the first solder block and the secondsolder block into a single integral solder bump alloy, wherein thesolder bump alloy has a specified composition ratio of a material thatconstitutes the first solder block to a material that constitutes thesecond solder block.
 15. The method of claim 14, wherein the step offorming an under-ball metallic layer over the active surface of thewafer includes the sub-steps of: forming an adhesion layer over theactive surface of the wafer; forming a barrier layer over the adhesionlayer; and forming a wettable layer over the barrier layer.
 16. Themethod of claim 15, wherein material constituting the adhesion layer isselected from a group consisting of titanium, titanium-tungsten alloy,aluminum and chromium.
 17. The method of claim 15, wherein materialconstituting the barrier layer is selected from a group consisting ofnickel-vanadium alloy, chromium-copper alloy and nickel.
 18. The methodof claim 15, wherein material constituting the wettable layer isselected from a group consisting of copper, palladium and gold.
 19. Themethod of claim 14, wherein each second solder block includes an uppersurface and a lower surface, the lower surface is in contact with theupper surface of the first solder block, and the upper surface of thesecond solder block is planarized by polishing.
 20. The method of claim14, wherein the material constituting the first solder blocks isselected from the group consisting of lead-tin alloy, lead-silver alloy,tin-silver alloy, silver and gold.
 21. The method of claim 14, whereinthe material constituting the second solder blocks is selected from thegroup consisting of lead-tin alloy, tin-silver alloy, tin-silver-copperalloy and tin.
 22. The method of claim 14, wherein the step ofplanarizing the upper surface of the first solder blocks includespolishing.
 23. The method of claim 14, wherein the step of planarizingthe upper surface of the first solder blocks includes applying apressure.
 24. The method of claim 14, wherein the step of attaching afirst solder block to the under-ball metallic layer includes thesub-steps of: providing a conductive wire; heating one end of theconductive wire so that the heated end of the conductive wire transformsinto a spherical blob; pulling the spherical blob towards the under-ballmetallic layer and pressing the spherical blob against the surface ofthe under-ball metallic layer; and detaching the remaining portion ofthe conductive wire from the spherical blob to form the first solderblock.
 25. The method of claim 24, wherein the step of pressing thespherical blob against the under-ball metallic layer is further assistedby application of ultrasound.
 26. The method of claim 14, wherein thestep of attaching a second solder block to a first solder block furtherincludes the sub-steps of: providing a conductive wire; heating one endof the conductive wire so that the heated end of the conductive wiretransforms into a spherical blob; pulling the spherical blob towards thefirst solder block and pressing the spherical blob against the uppersurface of the first solder block; and detaching the remaining portionof the conductive wire from the spherical blob to form the second solderblock.
 27. The method of claim 26, wherein the step of pressing thespherical blob against the first solder block is further assisted byapplication of ultrasound.
 28. A method of forming bumps on the activesurface of a silicon wafer, wherein the active surface further includesan under-ball metallic layer thereon and the under-ball metallic layeris a composite layer that comprises a plurality of material layers, themethod comprising the steps of: attaching at least one first solderblock to an upper surface of the under-ball metallic layer by conductinga bonding operation; attaching at least one second solder block to theupper surface of the first solder block by conducting a bondingoperation; and alloying at least one of the first solder block and atleast one of the second solder block into at least one single solderbump.
 29. The method of claim 28, wherein after the step of attachingthe first solder block to the upper surface of the under-ball metalliclayer, further includes planarizing the upper surface of the firstsolder block and attaching a second solder block to the upper surface ofthe first solder block by conducting a bonding operation.
 30. The methodof claim 29, wherein the step of planarizing the upper surface of thefirst solder block includes polishing.
 31. The method of claim 29,wherein the step of planarizing the upper surface of the first solderblock include pressing.
 32. The method of claim 28, wherein after thestep of attaching the second solder block to the upper surface of thefirst solder block, further includes planarizing the upper surface ofthe second solder block.
 33. The method of claim 32, wherein the step ofplanarizing the upper surface of the second solder blocks includespolishing.
 34. The method of claim 28, wherein material constituting thefirst solder blocks is selected from the group consisting of lead-tinalloy, lead-silver alloy, tin-silver alloy, silver and gold.
 35. Themethod of claim 28, wherein material constituting the second solderblocks is selected from the group consisting of lead-tin alloy,tin-silver alloy, tin-silver-copper alloy and tin.
 36. The method ofclaim 28, wherein the step of attaching a first solder block to theunder-ball metallic layer includes the sub-steps of: providing aconductive wire; heating one end of the conductive wire so that theheated end of the conductive wire transforms into a spherical blob;pulling the spherical blob towards the under-ball metallic layer andpressing the spherical blob against the surface of the under-ballmetallic layer; and detaching the remaining portion of the conductivewire from the spherical blob to form the first solder block.
 37. Themethod of claim 36, wherein the step of pressing the spherical blobagainst the under-ball metallic layer is further assisted by applicationof ultrasound.
 38. The method of claim 28, wherein the step of attachinga second solder block to a first solder black further includes thesub-steps of: providing a conductive wire; heating one end of theconductive wire so that the heated end of the conductive wire transformsinto a spherical blob; pulling the spherical blob towards the firstsolder block and pressing the spherical blob against the upper surfaceof the first solder block; and detaching the remaining portion of theconductive wire from the spherical blob to form the second solder block.39. The method of claim 38, wherein the step of pressing the sphericalblob against the first solder block is further assisted by applicationof ultrasound.